The ABCD rule introduced by Stolz and coworkers [Eur J Dermatol 1994] can be easily learned and rapidly calculated and has been proven to be a reliable method providing a more objective and reproducible diagnosis of melanoma. Also, in 1994, Nachbar et al. [J Am Acad Dermatol 1994] proved the reliability of the ABCD rule in a prospective study. In 172 melanocytic lesions (69 melanomas and 103 melanocytic nevi) specificity was 90.3% and sensitivity was 92.8%. One may argue, however, that the pretest probability with 69 melanomas out of 172 melanocytic lesions was much too high in this prospective study and does not reflect the real scenario even in a specialized pigmented skin lesion clinic.
A given melanocytic lesion is bisected by two 90° axes that were positioned to produce the lowest possible asymmetry score. If both axes show dermocopically asymmetric contours with regard to colors and differential structures, the asymmetry score is 2. If there is asymmetry on one axis the score is 1. If asymmetry is absent with regard to both axes the score is 0. Remarkably, most melanomas have an asymmetry score of 2 compared to about only 25% of benign melanocytic nevi. By using dermoscopy, asymmetry can be more precisely evaluated. Indeed, colors and structures are much better visible compared to the naked eye which, in most instances, allows assessment of asymmetry only by contour. Because of its high (1.3) weight factor, the assessment of asymmetry is crucial for the final score and one should also keep in mind that in a strict sense ‘nothing in nature is completely symmetric’.
For semiquantitative evaluation, the lesions are divided into eighths and a sharp, abrupt cut-off of pigment pattern at the periphery within one eighth has a score 1. In contrast, a gradual, indistinct cut-off within one eighth has a score of 0. So, the maximum border score is 8, and the minimum score is 0. As a rule the border score in nevi is very low and in melanomas is predominantly between 3 and 8. Because of its low weight factor (0.1) the border score is not very relevant, at least in our view.
A total number of six different colors, namely, white, red, light-brown, dark-brown, blue-gray, and black, are counted for determining the color score. White should be only chosen if the area is lighter than the adjacent skin. When all six colors are present the maximum color score is 6; the minimum score is 1. Melanomas are usually characterized by three or more colors and in about 40% of melanomas even five or six colors are present. Remarkably, the color spectrum of melanocytic lesions is accentuated and intensified when performing dermoscopy.
The following five structural features have been selected by Stolz for evaluation of differential structures: pigment network, structureless or homogeneous areas, streaks, dots, and globules. Basically all these criteria have been explained in detail in step 2 of this course. Structureless or homogenous areas must be larger than 10% of the lesion. Streaks and dots are counted only when more than two are clearly visible. For counting a globule only the presence of one single globule is necessary. Again, the higher the number of these differential structures, the higher the probability of the lesion being a melanoma.